Origin of the Invention
The present invention was made under National Science Foundation Grant No. CEE-8119962 and is subject to the provisions of the National Science Foundation Act.
Because of cost, many ground supported fluid storage tanks are not anchored to their foundations, even in seismic areas. This is especially true for large capacity, broad tanks. When such an unanchored tank is subjected to strong ground shaking, the lateral force due to hydrodynamic pressures acting on the tank wall is of the same order of magnitude as the weight of the liquid. Unless a portion of the tank wall uplifts, the overturning moment induced by this lateral force can only be balanced by the stabilizing effect of the weight of the tank. For typical steel tanks the weight of the tank is much less than the weight of the contained liquid. Therefore, the weight of the tank is insufficient to balance the overturning moment due to hydrodynamic pressures acting on the tank wall, and the tank wall uplifts locally. As a result, a crescent-shape strip of the base plate is also lifted from the foundation. The weight of fluid resting on the uplifted portion of the base plate then provides the resisting moment against further uplift.
Unanchored tanks are special in that only the weight of fluid resting on the uplifted portion of the base plate contributes to the stabilizing moment, whereas the entire mass of fluid contributes to the overturning moment. This is different from the usual case in which the entire weight of a structure and its contents contributes to the stabilizing moment. As a result, unanchored fluid storage tanks are particularly prone to uplift problems.
Evidence of uplift can be found in the 1964 Alaska earthquake, during which snow found its way underneath the base plate of some tanks and during the 1971 San Francisco earthquake, when an anchor bolt of a 30 ft. tall and 100 ft. diameter tank was pulled up by 14".
The previously known established method of analysis for unanchored tanks is that developed by Wozniak, R. S. and Mitchell, W. W. (1978), "Basis of Seismic Design Provisions for Welded Oil Storage Tanks," Advances in Storage Tank Design, API, 43rd Midyear Meeting, Toronto, Ontario, Canada, however this work does not generally provide comprehensive methods for analysis of unanchored tanks and validation by experimental results. Various solutions to the problem of earthquake resistance in tanks have been suggested in U.S. Pat. No. 3,977,140 to Matsudaira, et al, U.S. Pat. No. 4,249,352 and U.S. Pat. No. 4,267,676 to Marchaj. However, these prior art patents all relate to bonding means or dampers and do not relate to unanchored storage tanks.
Although uplift itself is not necessarily associated with serious damage, it can be accompanied by large deformations and by major changes in the stresses in the shell of a tank. Experience in earthquakes has shown that the consequences of large uplift can include: (i) damage and breakage of connecting pipes; and (ii) buckling of the tank wall because the vertical compressive stress in the portion of the wall which remains in contact with the ground on the other side of the tank is greatly increased.